Multiplex phase discrimination transmission system



April 19, 1932. ls. BRAND ET AL y v MULTIPLEX PHA-SE DISCRIMINATION TRANSMISSION SYSTEM Filed oct. so, 192e Patented Apr. 19, 1932 UNITEDl S'luarlss, PATENT, oFFlcEi-.

SMART BRANI-D, F BROOKLYN, AND PIERRE MYERTZ, OF BELLEROSE MANOR, NEW YORK,

ASSIGNOBS TO AMRICAN TELEPHONE AND- TELEGRAPH COMPANY, A CORPORA-- TION OF NEW YORK MULTIPLEX- PHASE DISCRIMINATION TRANSMTSSION SYSTEM Application led October, 30, 1928. Serial No. 315,995.

This-invention relates to high frequency signaling systems and more particularly to diplex transmission systems whereby two independent message channels may be obtained from one high frequency channel by means of phase discrimination.

In the prior art, number of different types of systemsfor obtaining a plurality of message channels from a single high frequency channel by means of phase discrimination. One of such types of systems is illustrated in a Patent No. 1,608,566

to R. K. Potter, November 30, 1926. lIn this patent, the diplex phase discrimination systcm disclosed is utilized for the bihaural transmission of sounds. It is pointed out that this is but one of the many uses to which such a diplex transmission system as is disclosed therein could be put. It is pointed out that a system of the type disclosed in said patent is capableof diplex use in general. ln other words, it might be utilized for the transmission ofmessages from two separate sources rather than from a single source. The above mentioned Vpatent is referred to only as an illustration of a specific method of diplex transmission utilizing phase discrimination purposes and capable of other uses than for binaural transmission. Accordingly, the arrangements of the invention are applicable to multiplex phase discrimination systems in general, and in particular to systems for bnaural transmission.

It is pointed out that in systems of the type illustrated in the above patent it is essential that the carrier currents used for detection in the receiving set, whether transmitted or generated locally. should have denite, constant phase relations with the carrier currents used at the transmitting station. One of the methods in the above mentioned types of systems has been to employ for this purpose the usual component of the carrier frequency in the transmitted output. This component will be the resultant of two similar components 90 apart in phase. one for each of the two independent message channels. Howover. it has been found that there are important limitations in the use of said carrier component in this connection. This 1s due to the there are. to be found a fact that at the receiving statio'n the lcircuit used for selecting the carrier component cannot be-made infinitely sharp,`and will thus allow slow variation in the amplitude of the carrier component, correspondingl to the modulation from the lower frequencies of the voice. Since the two channels are independent the components of the carrier frequency.

in these will vary .in magnitude independently. This will vary the phase of the resultant with' respect to either of the carrier compophase angle be constant. This effect is more pronounced as the modulation is reater--the limitation therefore consists in t e necessity of keeping the modulation small. Accordingly, the primary object of the arrangement of this invention is to provide a novel method for transmitting current to perform this function when amplified at the receiving station,

which will avoid the limitations existin in former systems. Other features and obJects of the invention will appear more fully from the detailed description hereinafter given.

In the method of this invention, a certain amount of entirely unmodulated carrier current is superimposed on the regular modulated current. To prevent confusion betweenl carrier is separated from the rest of the current by a sharply tuned circuit and passed through a phase shifter and amplifier to the phase discrimination apparatus for purposesv of detecting.

The inventlon may be more fully under-l stood from the ,following description, to

gether with the accompanying drawings, in the Figures 1 and 2 of which the invention is illustrated. Fig. 1' is a circuit diagram of a radio transmitting station, and Fig. '2 is a circuitdiagram of a radio receiving station embodying principles of the invention.

In Fig. l are shown two transmitters T1 nents,thus violatingthe requirement th at such and T2 connected to circuits 1 and 2, whereby the separate channels A and B may be transmitted to the tubes M1 and M2 of a modulator of the well known 'duplex type. Associated With the modulator is an oscillator 2. Current from this oscillator of carrier frequencies is passed through a resistance 3 and a condenser4 in series. rl`he potential dierences across these will be 90'degrees out of phase with each other. @n one and the other of these, respectively, are superimposed the modulating currents from channels A and B. After modulationV the currents from the two channels will be transmitted throughthe band suppression filter 6 and thence to the sending antenna 7 or any other desirable type of transmitting circuit. rlhe band suppression lter will eliminate current o'f the car-l be received by a suitable device such as thev antenna 8 and amplified by the tube 9. The unmodulated carrier current will be Separated from the rest of the current by a sharply tuned circuit 10. The width of the bandsuppressed by filter 6 should be greater than that of the band transmitted by the tunedrcircuit 10 so that no current of frequencies in the side-bands resulting from the modulation in tubes M1 and M2 will pass through the tuned circuit 10. The unmodulated carrier current will then be passed through the ampliier'll, phase shifter 12, and amplifier 13 to the phase discrimination apparatus comprising the dey tectortubes DM1, DM2, and the input circuits associated-therewith which have the resistance 15 and condenser 16 connected in series. The rest of the 'signaling currents will be transmitted overthe circuit 14 to the detector apparatus. These currents will be detected by the phase/discrimination apparatus in a Well known mannerand channels A and B will be transmitted over the circuits 17 and 18, respectively, to the receivers 19 and 20.

The theory of operation of the principles of a hase discrimination system is explained brie ybelow.

The general principles ofY operation of a two-channel radio telephone usystem using phase discrimination are as follows:

Let the'voice current `from channel A ink press ton its appropriate grid a voltage 'v1 f (t) The carrier current ifi-ril sin wt where w=21r carrier frequency will impress on the same grid a voltage o2= V sin wt so 4,that the total grid voltage for channel A, including the C battery voltage, will be The plate current of a vacuum tube is,-in terms ofthe grid voltage,

fr (a3-cy where-l0 and 0 are constants characteristic of the tube. In the present case, then Fav sin www) +0212 k= V2 8h12 www sin @mauve 12+2Vc2 sin we+2fc a+c22 y'lhe voltage output at the plate will be sub- Assuming these very hi h andy very low frequency terms to be neg igible, either due to the inefficiency of. the transformer or to appropriate filters kf=wc2+ We] sin wv rlhis may be written a5= [ad-HU] sin wt where o5 is now simply proportional to the voltage instead of equal to it.

Considering channel B, the voice current will impress on the grid a voltage The same carrier current as before, but in the if The plate voltage will be proportional to output transformer voltage will be proportional to fi fvllo=[b'+ig(t)] cos wt. The total voltage from ,channels A and B superimposed will be.

In the antenna, if the carrier frequency itself is not suppressed, the voltage will be proportional to this (the case where the carrier frequency is suppressed corresponds to the special case a=b=o). 4

The receiving modulated carrier voltage will also be proportional to this. For channel A there is superimposed upon this'voltage The second factor above, in so far as it is vdif` ferent from 1, represents interference from channel B. It Will be noted that the lirst order term is missing, and that the second order term may be reduced by making A large in comparison with the other quantities,

and by making a and bsmall. rl`he amplitude A may be madelarge by the use of amplifiers., as shown in the drawings.

The voltage after rectification Willobe the envelope of the above, or

@1f-*A -l-ajl-U) if the interference is negligible. Upon re moving the constant component, there remains J.

U15 f t) which is impressed upon the telephone receiver. A similar demonstration would follow for theB channel.

The component of carrier frequency in the transmitted output, when not suppressed, can

" be used for securin the necessary phase rela'- tions at the receiving end, only with limitations, as shown below.

The output voltage from the transmitting antenna is rEhe received voltage will Vbe the above Which will furnish'the detecting current by passing through a device 'arranged to give a constant amplitude (say an overloaded ampliiie'r) or giving The phase of this voltage will vary with time. The range of variation of this phase Will depend upon a, b, and the range of variation of f(t) andg(t)'. The usual and interestingcase is When both channels A and B are substantially similar, so that a=b and f(t) and g(t) var)7 between the same limits, say betweenl m and m.- Then the range of phase variation Will be or approximately rlihe rlaylor series for the arc tangent ex-A panded about the value 1 is 2m` Il -j radlans.

rl`hus-it is necessary under these circum'- stances to keep the modulation small, i. e., m small compared With a. Assuming that the range 0f phase variation should be kept with +5", then a must be made more than about 24 times as large as m (since onlylirst order terms have been kept the expression is not very accurate unless \I1 is small).

This invention is in part a continuation of application Serial No. 230,877, filed Novcmber 3, 1927, in the name of Smart Brand and Pierre Mertz.

While the invention has been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood that it is capable of embodiment in many and other widely varied forms Without departingI from the spirit of the invention as deiined by the appended claims. 1

wWhat is claimed is:

seal u 1. In a system for the binaural transmission of signals, the method which consists in the modulation of a carrier by signal varia,- tions resulting from sound waves to produce side bands and a modulation component of carrier frequency, separately modulating by another set of signal variations resulting from sound waves from the same source carrier oscillations of the same frequency as employed in the first mentioned modulation 0peration but differing in phase by 90 therefrom to produce side bands and a modulation component of carrier frequency, eliminating the resultant of both modulation components of carrier frequency, and transmitting both sets of side bands and a monophase current of carrier frequency.

2. In a system for the binaural Vtransmission of signals, the method which consists in the modulation of a carrier by signal variations resulting from sound waves to produce side bands and a modulation component of carrier frequency, separately modulating by another set of signal variations resulting from sound waves from the same source carrier oscillations of the same frequency as employed in the kfirst mentioned modulation ,op-

eration but differing in phase by 90 therefrom to produceside bands and a modulation component of carrier frequency, eliminating the resultant of both modulation components of carrier frequency, transmitting both sets of side bands and a monophase current of carrier frequency to a receiving station, se-

' lecting said monophase current of carrier frequency from said side bands at said receiving station, and utilizing said current of carrier frequency for phase discrimination de.

tection of said side bands.

3. In a system for the-binaural transmis- 4sion of signals a'transmitting station having means to separately modulate carrier oscillations by the signal variations resulting from sound from the same source, the carrier oscillations being of the same frequency but differing in phase by degrees, thereby producing side bands and two modulation comonents of carrier frequency, means to eliminate from the transmitted output the result- `ant of the two modulationl components of carrier frequency, andmeans to apply to the crimination detection.

transmittedoutput a monophase current of carrier frequency for purposes of phase dis- 4. In a system for the binaural transmission of signals the combination with a transmitting station having means to separately modulate carrier, oscillations by the signal variations resulting from sound from the same source, the carrier oscillations being of i the same frequency but differing in phase by 90 degrees, thereby producing'side bands and e two modulation components of carrier frequency, means to eliminate from the transmitted output the resultant of the two mbdulation-components of carrier frequency, means-- to apply to the transmitted-output a monophasencurrent of carrier frequency, of a receiving station including a phase discrimination detecting device, a circuit for selectively applying to said detecting device the received side bands, and a circuit for' selectively applying to said detecting device the received monophase current of carrier frequency.

5. 1n a system for the multiplex transmission of signals, the method which consists in the modulation of a carrier by signal variations to produce side bands and a modulaa monophase current of carrier frequency'.`

6. fn a system for the multiplex transmission of signals, the method which consists in the modulation of a carrier by Vsignal variations to produce side bands anda modulation lcomponent'of carrier frequency, separatelyl 'modulating by another set of signal variations carrier oscillations of the same frequency as employed in the rstmentioned modulation operation but differing in phase by 90 therefrom to produce side bands and a modulation component of carrier frequency, eliminating the resultant of both modulation components of carrier frequency, transmitting both sets of side bands and a monophase current'of carrier frequency to a receiving station, selecting said carrier frev quency from said side bands at said receiving station, and -utilizing said carrier frequency for phase discrimination detection of said side bands. l

7. In a system for the multiplex transmission of signals a transmitting station having means to separately modulate carrier oscillations by signal variations, the carrier oscillations being of the same. frequency but dif-l fering in phase by 90 degrees, thereby producing side bands and two modulation components of carrier frequency, means to eliminate from the transmitted output the resultant of the two modulation outputs of carrier frequency, and means to apply to the transmitted output a monophase current of carrier frequency oscillations for purposes of phase discrimination detection. l

8. ln asystem for the multiplex sion of signals the combination with a transmitting station having means to`separately modulate carrier oscillations by signal variations, the' carrier oscillations being of the 'saine frequency but differing in phase by 90 degrees,thereby producing-side bands and two modulation components of carrier fretransmisquency, means to eliminate from the transmitted output the resultant of the two modulation components of carrier frequency, means to apply to the transmitted output a monophase current of carrier frequency, of a receiving station including a phase discrimination detecting device, a circuitl for selectively appl ing to said detecting device the received sid'e bands, and a circuit for selectively applying to said detecting device the l0 received monophase curre tof carrier frequency.

In testimony whereof, We have signedl our names to this specification this 29th day of October, 1928.

1 SMART BRAND.

PIERRE MERTZ. 

